My Samsung Galaxy S2 on Gingerbread was set to use Handcent as the default ringtone picker, and I accidentally changed this back to the built-in ringtone picker. How do I get it to use Handcent again? I went to Handcent, cleared the checkbox and marked it back again, but it didn't help.

After this, When you are choosing a ringtone, You will be presented with a dialog box asking you to select one of the application as ringtone picker. Enable the 'Use by default for this action' Check-box and select Handcent. This will set Handcent as your default ringtone picker.

One workaround would be to temporarily install another app that has a ringtone picker in order to show the ringtone picker selection screen, select Handcent, and remove the temporary app. For example, you can try Zedge.